1. Field of the Invention
The present invention relates to high accuracy diffraction interferometry, and more specifically, it relates to the use of embodiments of the Phase Shifting Interferometer to measure the aberrations of convex lenses and negative lenses.
2. Description of Related Art
Interferometry is the preferred method to measure the performance of optical elements and systems. In this method the wavefront of light reflected from or transmitted by the optic to be tested is interfered with the wavefront from a reference surface, to produce an interference fringe pattern. These interference fringes are then analyzed to ascertain the performance of the optic. For high performance imaging systems, such as those found in lithographic steppers used to make integrated circuits, this interferometric measurement must be made to ever increasing accuracy. The accuracy, however, is limited by how well the reference surface is characterized. Reference surfaces are typically no better than xe2x96xa1/50, where xe2x96xa1 is the wavelength of visible light, and thus are the limiting factor in fabricating higher performance optical systems. Therefore the fabrication of high accuracy optical systems, such as those needed for extreme ultraviolet projection lithography which require an accuracy of xe2x96xa1/1000, are impossible to qualify with confidence using existing interferometry.
The problem or difficulty with interferometrically measuring a convex mirror or a negative lens is that it is necessary to have a converging spherical wavefront incident on either of these two optics in order to make a measurement with an interferometer. This problem is particularly true of the phase measuring diffraction interferometer since it produces a perfect diverging spherical wavefront.
In order to produce a converging spherical wavefront, it is necessary to introduce a positive auxiliary lens into the interferometer. This will change the perfect diverging spherical wavefront into a converging wavefront. This converging wavefront will not, in general, be a perfect spherical wavefront due to errors in the positive auxiliary lens. This will introduce an error into the measurement. In conventional interferometers this error cannot be eliminated. However, the phase shifting diffraction interferometer is unique in that it can be configured in at least two different ways, permitting exact cancellation of the error.
It is an object of the present invention to provide methods utilizing the phase shifting diffraction interferometer for measuring the aberrations of convex mirrors and negative lenses.
Other objects of the invention will be apparent to those skilled in the art based on the teachings herein.
U.S. Pat. Nos. 5,548,403 and 5,933,236, disclose an interferometer that has the capability of measuring optical elements and systems with an accuracy of xe2x96xa1/1000 where xe2x96xa1 is the wavelength of visible light. Whereas current interferometers employ a reference surface, which inherently limits the accuracy of the measurement to about xe2x96xa1/50, this interferometer uses an essentially perfect spherical reference wavefront generated by the fundamental process of diffraction. This interferometer is adjustable to give unity fringe visibility, which maximizes the signal- to-noise, and has the means to introduce a controlled prescribed relative phase shift between the reference wavefront and the wavefront from the optics under test, which permits analysis of the interference fringe pattern using standard phase extraction algorithms. The patented interferometers maximize the signal-to-noise and permit analysis of interference fringe patterns using standard phase extraction algorithms.
The measurement of convex mirrors and negative lenses may be accomplished though the introduction of auxiliary interferometer optics into certain embodiments of the phase shifting interferometers described in the above described patents.
To measure a convex mirror, the reference beam and the measurement beam are first both provided through a single optical fiber. A positive auxiliary lens is placed in the system to give a converging wavefront onto the convex mirror under test. An aperture stop is located immediately after the positive lens. The aperture stop defines where the convex mirror to be tested will be located. The convex mirror is placed right at the aperture stop in such a way that the light is reflected back on itself from the surface of the convex mirror. The wavefront reflected from the convex mirror under test comes to focus on the end of the fiber where it is reflected off the face of the fiber and is combined with the wavefront coming directly out of the fiber. Both wavefronts are imaged onto a CCD camera. An interference pattern is observed at the CCD camera and recorded (stored). The interference pattern is analyzed by standard methods. This constitutes one of the measurements. This measurement includes the aberrations of the convex mirror as well as the errors due to two transmissions through the positive auxiliary lens. A second measurement provides the information to eliminate this error.
To make the second measurement, the first fiber, imaging lens, CCD camera, and aperture stop are left in exactly the same positions. It is important that they are not moved between the two measurements. The convex mirror under test is removed. A second optical fiber is placed at the focal position of the positive lens. For this second measurement, only the reference beam is provided through the original optical fiber and the measurement beam is provided through the second optical fiber. The measurement wavefront from the second optical fiber passes through the aperture stop, goes through the positive auxiliary lens and comes to focus on the face of the original optical fiber. It then reflects off the face of the original optical fiber and is combined with reference wavefront coming directly out of the first fiber. Both wavefronts are imaged onto the CCD camera. The interference pattern is analyzed by standard methods. This constitutes the second measurement. This measurement includes only errors due to a single transmission through the positive auxiliary lens. To obtain the aberration due to just the convex mirror, the second measurement is multiplied by 2 and the result is subtracted from the first measurement. An alternate embodiment for measuring a convex mirror is also provided.
A negative lens can also be measured in a similar way. Again, there are two measurement set-ups. The reference beam is provided from a first optical fiber and the measurement beam is provided from a second optical fiber. A positive auxiliary lens 130 is placed in the system to provide a converging wavefront to the reference beam. An aperture stop is placed immediately after the positive lens. The negative lens under test is placed at the aperture stop. The measurement fiber is placed at the focal position of the negative lens under test. The measurement wavefront passes through the negative lens and aperture stop, goes through the positive lens and comes to focus on the face of the reference optical fiber. It then reflects off the face of the reference optical fiber and is combined with the reference wavefront coming directly out of the reference fiber. Both wavefronts are imaged onto the CCD camera. The interference pattern located at the CCD camera is analyzed by standard methods. This constitutes one of the measurements. This measurement includes the aberrations of the negative lens, as well as the errors due to a single transmission through the positive auxiliary lens. A second measurement provides the information to eliminate this error.
To make the second measurement, the reference fiber, imaging lens, CCD camera, and aperture stop are left in exactly the same positions. It is important that they are not moved between the two measurements. The negative lens is removed. The measurement optical fiber is moved to the focal position of the positive auxiliary lens. The measurement wavefront from the measurement optical fiber passes through the aperture stop, goes through the positive auxiliary lens and comes to focus on the original optical fiber. It then reflects off the face of the original optical fiber and is combined with the reference wavefront coming directly out of the reference fiber. Both wavefronts are imaged onto the CCD camera. The interference pattern located at the CCD camera is analyzed by standard methods. This constitutes the second measurement. This measurement includes only errors due to a single transmission through the positive auxiliary lens. To obtain the aberration due to just the negative lens, the second measurement is subtracted from the first measurement. This eliminates the error due to the positive auxiliary lens and gives just the wavefront transmitted by the negative lens.